A magnetic recording medium such as a video tape, audio tape, memory tape, magnetic sheet or magnetic disk comprises a base film and a magnetic recording layer formed on the surface of the base film. On a surface opposite to the magnetic recording layer, a slippery back coat layer is formed in many cases to increase slipperiness. As the base film of the magnetic recording medium is mainly used a polyester film. The adhesion of the polyester film to the magnetic recording layer and the adhesion of the polyester film to the slippery back coat layer are important properties. If these adhesion properties are unsatisfactory, the magnetic recording layer and the back coat layer peel off and magnetic characteristics are completely lost in the sound recording, image recording or reproduction step of the magnetic recording medium.
Problems that the Invention Intends to Solve
There are known a large number of polyester base films having improved adhesion. They include, for example, a polyester base film whose surface is subjected to corona discharge, a polyester base film whose surface is coated with an adhesive resin and the like. In order to provide a marked adhesive effect, it is desired to coat the surface of a polyester base film with an adhesive resin.
In recent years, along with an increase in magnetic recording density, the surface of a polyester film used as a base film has been made less rough and more flat. In this case, blocking readily occurs in the roll of a conventional polyester base film coated with an adhesive resin and the film is easily broken or torn when it unrolled in the production process of a magnetic medium.
Particularly, in a vacuum-deposited tape having a magnetic recording thin film layer on the surface of a polyester base film, such as a deposited video tape, the polyester base film has a low surface roughness and hence, the slipperiness of the tape traveling surface must be improved by forming a back coat layer on a side opposite to a magnetic recording layer. When the back coat layer is coated with an adhesive resin by a conventional technology to improve the adhesion of the back coat layer to the polyester film, blocking readily occurs because the surface roughness of the polyester base film is extremely low.
It is considered that this blocking is caused by the fact that moisture contained in the air permeates into the surface of the film or penetrates between surfaces of films and the surfaces of the films become a state that they are adhered to each other by pressure between the films. Although film rolls after film production or before use are kept at low humidity at a plant and it is possible to prevent blocking to a certain degree by strictly controlling storage conditions, there is no radical solution to this problem. Particularly, in the case of a polyester film for a deposited magnetic recording medium, it is impossible to prevent blocking of an adhesive film by the control of humidity alone.
A blocked rolled film may be broken when it is unrolled, or even if it is not broken, a material of a coating layer or polyester film itself may be transferred to a contacting surface by local blocking, and a dropout may be produced when a tape is formed from such a film.
A polyester film which is readily blocked is easily electrified and a high electrostatic film involves such problems that the handling properties of the film greatly deteriorate at the time of film formation and tape formation, sparks generated by electrostatic charge may ignite an organic solvent used for the formation of a tape, the film easily adsorbs suspending dust in the air electrically, and in particular, the dust causes a dropout in a deposited tape and the like which require high-density recording.
It is an object of the present invention to provide a low electrostatic polyester film whose amount of electrostatic charge is small and which is free from blocking between films, rarely experiences the transfer of a material to an opposite side of a coating layer caused by blocking and is suitable for use as a base film for a high-density magnetic recording medium having excellent electromagnetic conversion characteristics, dropout resistance and adhesion of a back coat when it is used in a magnetic recording medium.
Means for Solving the Problems
According to the studies conducted by the inventors of the present invention, it has been found that the above object of the present invention can be attained by a low electrostatic composite polyester film having the following features (1) to (3), which comprises a base film C, a coating layer A formed on one side of the base film C and a coating layer B formed on the other side of the base film C.
(1) The coating layer A is formed of a water-soluble or water-dispersible resin containing inert particles having an average particle diameter of 5 to 100 nm and has protrusions at a density of 1.times.10.sup.6 to 1.times.10.sup.8 /mm.sup.2 on the surface and a center line average roughness (Ra-A) of the surface of 0.1 to 2 nm.
(2) The coating layer B comprises 1 to 40 wt % of inert particles having an average particle diameter of 20 to 100 nm and 60 to 99 wt % of a composition containing a water-soluble or water-dispersible resin and the resin-containing composition contains 5 to 85 wt % of a silicone-modified polyester resin or 1 to 30 wt % of silicone or wax based on the layer B.
(3) The base film C is an aromatic polyester film which may contain no inert particles or may contain inert particles having an average particle diameter of 5 to 2,000 nm in an amount of 0.001 to 5.0 wt %.
The present invention is a low electrostatic composite polyester which comprises at least three layers: a polyester base film C, a coating layer A formed on one side of the polyester film C and a coating layer B formed on the other side of the polyester film C and has the improved property of suppressing charging with static electricity by means of the coating layer A and/or the coating layer B and blocking between films.
The low electrostatic composite polyester film of the present invention will be described in detail hereinafter.
The aromatic polyester forming the core layer C of the composite film of the present invention is selected from polyethylene terephthalate, polyethylene isophthalate, polytetramethylene terephthalate, poly-1,4-cyclohexylene dimethylene terephthalate, polyethylene-2,6-naphthalene dicarboxylate and the like. Of these, polyethylene terephthalate and polyethylene-2,6-naphthalene dicarboxylate are preferred.
The above polyester may be either a homopolyester or a copolyester. Copolymer components that are copolymerizable with polyethylene terephthalate and polyethylene-2,6-naphthalene dicarboxylate include diol components such as diethylene glycol, propylene glycol, neopentyl glycol, polyoxyethylene glycol, p-xylene glycol and 1,4-cyclohexanedimethanol; other dicarboxylic acid components such as adipic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid (for polyethylene-2,6-naphthalene dicarboxylate), 2,6-naphthalenedicarboxylic acid (for polyethylene terephthalate) and 5-sodium sulfoisophthalic acid; oxycarboxylic acid components such as p-oxyethoxybenzoic acid; and the like. The amount of the copolymer component is preferably 20 mol % or less, more preferably 10 mol % or less, based on the total of all dicarboxylic acid components.
Further, a polyfunctional compound having a 3 or more functional groups, such as trimellitic acid or pyromellitic acid, may be copolymerized. In this case, it can be copolymerized in such an amount that the polymer is substantially linear, for example, in an amount of 2 mol % or less.
The polyester film as the base film C in the present invention may or may not contain inert particles. When it contains inert particles, the inert particles may be either organic or inorganic. As will be described later, the inert particles to be contained in the base film C may be different from the inert particles contained in the coating layer A and the coating layer B in type and average particle diameter. Illustrative examples of the organic inert particles include core-shell structured particles such as crosslinked polystyrene, polystyrene-divinylbenzene copolymer, polymethyl methacrylate, methyl methacrylate copolymer, methyl methacrylate crosslinked copolymer, polytetrafluoroethylene, polyvinylidene fluoride, polyacrylonitrile, benzoguanamine resin and graft copolymers comprising these polymers. Illustrative examples of the inorganic inert particles include silica, alumina, titanium dioxide, feldspar, kaolin, talc, graphite, calcium carbonate, molybdenum disulfide, carbon black and barium sulfate. These particles can be added to a reaction system, preferably as a slurry contained in glycol, during the production of a polyester, for example, at any time during an ester interchange reaction or a polycondensation reaction when it is produced by an ester interchange method, or at any time when it is produced by a direct polymerization method. The average particle diameter of the inert particles is preferably 5 to 2,000 nm, more preferably 10 to 1,800 nm and the amount of the inert particles is preferably 0.001 to 5 wt %, more preferably 0.001 to 2 wt %, particularly preferably 0.01 to 1.5 wt % based on the polyester.
The base film C has a thickness of 1 to 30 .mu.m, referably 3 to 25 .mu.m.
A description is subsequently given of the coating layer A formed on the surface of one side of the base film C as a composite component.
Illustrative examples of the water-soluble or water-dispersible resin forming the coating layer A of the present invention include acrylic resins, polyester resins, acryl-polyester resins, alkyd resins, phenol resins, epoxy resins, amino resins, polyurethane resin, vinylacetate resins, vinyl chloride-vinylacetate copolymer and the like. From the viewpoint of the adhesion to an aromatic polyester, protrusion retainability and slipperiness of the base film C, acrylic resins, polyester resins and acryl-polyester resins are preferred. These water-soluble and water-dispersible resins may be either a homopolymer, copolymer or mixture.
The water-soluble and water-dispersible acrylic resins include, for example, acrylic acid esters (residual alcohol groups thereof include a methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, t-butyl group, 2-ethylhexyl group, cyclohexyl group, phenyl group, benzyl group, phenylethyl group and the like): methacrylic acid esters (residual alcohol groups thereof are the same as above); hydroxy-containing monomers such as 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate and 2-hydroxypropyl methacrylate; amide group-containing monomers such as acrylamide, methacrylamide, N-methyl ethacrylamide, N-methyl acrylamide, N-methylol acrylamide, N-methylol methacrylamide, N,N-dimethylol crylamide, N-methoxymethyl acrylamide, N-methoxymethyl ethacrylamide and N-phenyl acrylamide; amino group-containing monomers such as N,N-diethyl aminoethyl acrylate and N,N-diethyl aminoethyl methacrylate; epoxy group-containing monomers such as glycidyl acrylate, glycidyl methacrylate and allyl glycidyl ether; monomers containing a sulfonic acid group or salt thereof, such as styrenesulfonic acid, vinylsulfonic acid and salts thereof (such as sodium salts, potassium salts and ammonium salts thereof); monomers containing a carboxyl group or salt thereof, such as crotonic acid, itaconic acid, acrylic acid, maleic acid, fumaric acid and salts thereof (such as sodium salts, potassium salts and ammonium salts thereof); monomers containing an acid anhydride group such as maleic anhydride and itaconic anhydride; combinations of monomers such as vinyl isocyanate, allyl isocyanate, styrene, vinyl methyl ether, vinyl ethyl ether, vinyl trisalkoxysilane, alkyl maleic acid monoester, alkyl fumaric acid monoester, acrylonitrile, methacrylonitrile, alkyl itaconic acid monoester, vinylidene chloride, vinyl acetate and vinyl chloride. The water-soluble and water-dispersible acrylic resins containing a (meth)acrylic monomer such as an acrylic acid derivative or methacrylic acid derivative in an amount of 50 mol % or more are preferred, and those containing methyl methacrylate are particularly preferred.
The water-soluble or water-dispersible acrylic resin can be self-crosslinked with a functional group in the molecule or can be crosslinked using a crosslinking agent such as a melamine resin or epoxy compound.
Illustrative examples of the acid component forming the water-soluble or water-dispersible polyester resin used to form the coating layer A of the present invention include polycarboxylic acids such as terephthalic acid, sophthalic acid, phthalic acid, 1,4-cyclohexanedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, adipic acid, sebacic acid, dodecanedicarboxylic acid, succinic acid, 5-sodium sulfoisophthalic acid, 2-potassium sulfoterephthalic acid, trimellitic acid, trimesic acid, trimellitic anhydride, phthalic anhydride, p-hydroxybenzoic acid and monopotassium trimellitate. Illustrative examples of the hydroxyl compound component include polyhydroxy compounds such as ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, 1,4-cyclohexane dimethanol, p-xylylene glycol, adduct of bisphenol A with ethylene oxide, diethylene glycol, triethylene glycol, polyethylene oxide glycol, polytetramethylene oxide glycol, dimethylolpropionic acid, glycerin, trimethylol propane, sodium dimethylol ethylsulfonate, potassium dimethylolpropionate. Polyester resins can be produced from these compounds in accordance with a common used method. To produce an aqueous coating, an aqueous polyester resin containing a 5-sodium sulfoisophthalic acid component or carboxylate group is preferably used. The polyester resin can be self-crosslinked with a functional group in the molecule or can be crosslinked using a curing agent such as a melamine resin or epoxy resin.
The water-soluble or water-dispersible acryl-polyester resin used to form the coating layer A of the present invention comprehends both acryl-modified polyester resins and polyester-modified acrylic resins and is formed by bonding the above acrylic resin component and the above polyester resin component together and exemplified by graft-type and block-type resins. The acryl-polyester resin can be produced by adding a radical initiator to both ends of a polyester resin to polymerize an acrylic monomer, adding a radical initiator to the side chain of a polyester resin to polymerize an acrylic monomer, or adding a hydroxyl group to the side chain of an acrylic resin to react it with a polyester having an isocyanate group or carboxyl group at a terminal so as to form a comb-shaped polymer.
The coating layer A contains inert particles (to be referred to as "inert particles A" hereinafter) which are made from an organic material such as crosslinked polystyrene, polystyrene-divinylbenzene, polymethyl methacrylate, methyl methacrylate copolymer, methyl methacrylate crosslinked copolymer, polytetrafluoroethylene, polyvinylidene fluoride, polyacrylonitrile or benzoguanamine resin, or an inorganic material such as silica, alumina, titanium dioxide, kaolin, talc, graphite, calcium carbonate, feldspar, molybdenum disulfide, carbon black or barium sulfate. A multi-layer-structured core-shell-type particle whose core and shell are made from materials having different properties may be used.
The inert particles A have an average particle diameter of 5 to 100 nm, preferably 10 to 50 nm. Further, the inert particles A preferably have a uniform particle size distribution. When the average particle diameter is smaller than 5 nm, slipperiness and abrasion resistance deteriorate. On the other hand, when the average particle diameter is larger than 100 nm, the particle falls off and abrasion resistance deteriorates. Further, since spacing between the magnetic head and the film becomes large, it is difficult to provide a high-density magnetic recording medium.
The inert particles A are contained in the coating layer A to ensure that the surface protrusion density should be 1.times.10.sup.6 to 1.times.10.sup.8 /mm.sup.2. When the surface protrusion density is lower than 1.times.10.sup.6 /mm.sup.2, the traveling durability of the resulting magnetic recording medium becomes insufficient. On the other hand, when the surface protrusion density is higher than 1.times.10.sup.8 /mm.sup.2, electromagnetic conversion characteristics are adversely affected. The surface protrusion density is preferably 2.times.10.sup.6 to 5.times.10.sup.7 /mm.sup.2, more preferably 3.0.times.10.sup.6 to 3.0.times.10.sup.7 /mm.sup.2.
The thickness of the coating layer A is in the range of 1 to 100 nm, preferably 3 to 70 nm. The ratio (t/d) of the thickness (t nm) of the coating layer A to the average particle diameter (d nm) of the inert particles A is in the range of 0.05 to 0.8, preferably 0.08 to 0.6, more preferably 0.1 to 0.5.
When this ratio (t/d) is larger than 0.8, the protrusion-forming function of the inert particles A lowers and the traveling durability of the resulting magnetic recording medium becomes insufficient. When the ratio is smaller than 0.05, particles on the surface of the laminate film are abraded by contacting the guide roll in the process of film formation, whereby traveling durability becomes insufficient and abraded particles are adhered to and accumulated on the film, with the result of an increase in the number of dropouts.
The coating layer A in the present invention is formed by applying a coating solution containing the above inert particles and the above water-soluble or water-dispersible resin onto at least one side of a polyester core layer and drying it. The solid content of this coating solution is 0.2 to 10 wt %, preferably 0.5 to 5 wt %, particularly preferably 0.7 to 3 wt %. This coating solution may contain other components such as a surfactant, stabilizer, dispersant, UV absorber, thickener and the like in such amounts that do not impair the effect of the present invention.
The center line average roughness (Ra-A) of the coating layer A-forming surface is preferably 0.1 to 2 nm, more preferably 0.5 to 1.5 nm. When the value of (Ra-A) is larger than 2 nm, the electromagnetic conversion characteristics of the resulting metal thin film magnetic recording medium degrade. On the other hand, when the value of (Ra-A) is smaller than 0.1 nm, slipperiness markedly deteriorates, traveling durability becomes insufficient, and the film sticks to the magnetic head, thereby making sound from the tape. Therefore, the tape may not be able to be put into practical use.
In the composite polyester film of the present invention, the coating layer A is formed on the surface of one side of the base film C and the coating layer B is formed on the surface of the other side of the base film C. The coating layer B will be described in detail hereinafter.
The coating layer B comprises 1 to 40 wt % of inert particles having an average particle diameter of 20 to 100 nm and 60 to 99 wt % of a composition containing a water-soluble or water-dispersible resin. The resin-containing composition contains 5 to 85 wt % of a silicone-modified polyester resin or 1 to 30 wt % of silicone or wax based on the layer B.
The coating layer B comprehends (i) a coating layer containing a silicone-modified polyester resin (to be referred to as "coating layer B-1" hereinafter) and (ii) a coating layer containing silicone or wax (to be referred to as "coating layer B-2" hereinafter).
To help the understanding of the two coating layers, the coating layer B-1 and the coating layer B-2 will be described separately. A description is first given of the coating layer B-1.
The coating layer B-1 comprises 1 to 40 wt % of inert articles having an average particle diameter of 20 to 100 nm and 60 to 99 wt % of a composition containing a water-soluble or water-dispersible resin, and the resin-containing composition contains 5 to 85 wt % of a silicone-modified polyester resin based on the layer B-1.
The silicone-modified polyester resin in the coating layer B-1 is a water-soluble or water-dispersible resin and is a compound in which a silicone component and a polyester resin component are bonded together. This bonding system may be either graft-bonding or block-bonding, for example. Stated specifically, this silicone-modified polyester resin can be produced by adding a radical initiator to both terminals of a polyester resin to polymerize silicone, or by adding a hydroxyl group to the side chain of silicone to react it with a polyester having an isocyanate group or carboxyl group at a terminal so as to form a comb-shaped polymer.
Illustrative examples of the polyester resin component used for polymerization are the same as those listed for the polyester resin used in the coating layer A.
The silicone is a silicone compound having a chain component represented by the following formula and an epoxy group, amino group, hydroxyl group or other functional terminal group at a terminal: ##STR1##
[wherein R.sub.1 is --CH.sub.3, --C.sub.6 H.sub.5 or hydrogen atom, R.sub.2 is --CH.sub.3, --C.sub.6 H.sub.5, hydrogen atom or functional group (such as epoxy group, amino group or hydroxyl group), and n is 100 to 7,000]. In the present invention, the silicone compound is not necessarily a homopolymer but may be a copolymer or a mixture of several homopolymers. PA1 [wherein R.sub.1 is --CH.sub.3, --C.sub.6 H.sub.5 or hydrogen atom, R.sub.2 is --CH.sub.3, --C.sub.6 H.sub.5, hydrogen atom or functional group (such as epoxy group, amino group or hydroxyl group), and n is 100 to 7,000]. In the present invention, the silicone compound is not necessarily a homopolymer but may be a copolymer or a mixture of several homopolymers.
The weight ratio of the polyester resin component to the silicone component is 98:2 to 60:40, preferably 95:5 to 80:20. The content of the silicone-modified polyester resin in the coating layer B-1 is 5 to 85 wt %, preferably 20 to 80 wt %. When the content is smaller than 5 wt %, its effect is insufficient, thereby causing blocking or increasing the amount of electrostatic charge, while when the content is larger than 85 wt %, adhesion to a back coat degrades, the film is transferred to the contacting surface when rolled, or the contacting roll is stained when the film travels.
The coating layer B-1 may contain other water-soluble or water-dispersible resins or surfactant which have been described in the description of the coating layer A in such amounts that do not affect the effect of the present invention, in addition to the inert particles and the water-soluble or water-dispersible silicone-modified polyester resin. The surfactant is preferably a nonionic surfactant, particularly preferably a surfactant prepared by adding or bonding an alkyl alcohol, alkyl phenyl alcohol or higher fatty acid to polyethylene oxide. When the surfactant is added in an amount of 20 wt % or less, preferably 1 to 15 wt %, based on the coating layer B-1, a coating failure or cissing at the time of coating can be prevented advantageously.
Further, when a cellulose resin is added to the coating layer B-1 in an amount of 5 to 40 wt %, preferably 10 to 30 wt % based on the coating layer B-1, many small continuous wrinkles can be formed in the coating layer B-1, whereby the winding property of the film can be improved. Illustrative examples of the cellulose resin include ethyl cellulose, methyl cellulose, acetyl cellulose, acetoacetyl cellulose, nitrocellulose, cellulose acetate butyrate and the like.
A description is subsequently given of the coating layer B-2. The coating layer B-2 comprises 1 to 40 wt % of inert particles having an average particle diameter of 20 to 100 nm and 60 to 99 wt % of a composition containing a water-soluble or water-dispersible resin, and the resin-containing composition contains 1 to 30 wt % of silicone or wax based on the coating layer B-2.
The silicone contained in the coating layer B-2 is a silicone compound having a chain component represented by the following formula and an epoxy group, amino group, hydroxyl group or other functional terminal group at a terminal: ##STR2##
The wax may be petroleum wax, vegetable wax, mineral wax, animal wax, low molecular weight polyolefin or the like and is not particularly limited. Illustrative examples of the petroleum wax include paraffin wax, microcrystalline wax, oxide wax and the like. Illustrative examples of the vegetable wax include candelilla wax, carnauba wax, Japan wax, oricurie wax, cane wax, rosin-modified wax and the like.
The content of silicone or wax in the coating layer B-2 is 1 to 30 wt %, preferably 1 to 15 wt % based on the coating layer B-2. When the content is smaller than 1 wt %, blocking occurs and the amount of electrostatic charge increases, while when the content is larger than 30 wt %, adhesion of a back coat degrades, the film is transferred to the contacting surface when rolled, or the contacting roll is stained when the film travels.
Illustrative examples of the water-soluble or water-dispersible resin forming the coating layer B-2 include acrylic resins, polyester resins, acryl-polyester resins, alkyd resins, phenol resins, epoxy resins, amino resins, polyurethane resins, vinyl acetate resins and vinyl chloride-vinylacetate copolymer, as those listed for the water-soluble or water-dispersible resin for the coating layer A.
Of the above water-soluble and water-dispersible resins, at least one resin selected from the group consisting of acrylic resins, polyester resins and acryl-polyester resins is preferred, and a combination of these resins and a cellulosic resin is more preferred. Illustrative examples of the acrylic resins, polyester resins and acryl-polyester resins are the same as those listed for the coating layer A. By using the water-soluble or water-dispersible resin in conjunction with a cellulosic resin, many small continuous wrinkles can be formed in the coating layer B-2, whereby the winding property of the film can be improved. Illustrative examples of the cellulosic resin include ethyl cellulose, methyl cellulose, acetyl cellulose, acetacetyl cellulose, nitrocellulose, cellulose acetate butyrate and the like. The content of cellulose in the coating layer B-2 is 5 to 40 wt %, preferably 10 to 30 wt %, based on the coating layer B-2.
Both of the coating layers B-1 and B-2 contain inert particles. The inert particles contained in these coating layers (to be referred to as "inert particles B") have an average particle diameter of 20 to 100 nm, preferably 20 to 50 nm, and the content thereof is 1 to 40 wt %, preferably 5 to 30 wt %. When the average particle diameter of the particles is smaller than 20 nm or the content is smaller than 1 wt %, the winding property and conveyance property in the film formation process of the film become unsatisfactory.
On the other hand, when the average particle diameter is larger than 100 nm, the particles readily fall off from the coating film. When the content of the inert particles B contained in the coating layer B is larger than 40 wt %, the coating layer B is readily abraded due to a reduction in the strength of the coating layer B itself.
The inert particles B contained in the coating layer B may be either particles of an organic material such as crosslinked polystyrene, polystyrene-divinylbenzene, polymethyl methacrylate, methyl methacrylate copolymer, methyl methacrylate crosslinked copolymer, polytetrafluoroethylene, polyvinylidene fluoride, polyacrylonitrile or benzoguanamine resin, or particles of an inorganic material such as silica, alumina, titanium dioxide, kaolin, talc, graphite, calcium carbonate, feldspar, molybdenum disulfide, carbon black or barium sulfate, as those listed for the coating layer A. The inert particles may be core-shell-type particles.
The coating layer B has a thickness of 1 to 100 nm, preferably 3 to 70 nm.
The center line average roughness (Ra-B) of the coating layer B-forming surface is 1 to 30 nm, preferably 2 to 20 nm. When the content of cellulose is smaller than 5 wt % or the value of (Ra-B) is smaller than 1 nm, the winding property and conveyance property in the film formation process of the film become unsatisfactory or blocking readily occurs. On the other hand, when the value of (Ra-B) is larger than 30 nm, the coating layer B is easily abraded.
When the base film C is formed of a coextruded layer, the winding property and conveyance property in the film formation process of the film can be fully provided according to the type of inert particles to be contained in the surface layer (C.sub.B) on which the coating layer B is formed. Unlike the case where the base film C is a single-layer film, if the base film C is a multi-layer film, a cellulosic resin is not necessarily contained. As a matter of course, the cellulosic resin may be contained.
The inert fine particles contained in the base film C of the surface layer (C.sub.B) on which the coating layer B is formed consist of single type of particles or two or more types of particles which differ in size. The average particle diameter of the particles of single type and the largest particles among two or more types of particles is 100 to 1,000 nm, preferably 100 to 500 nm. The content of the particles is 0.001 to 5.0 wt %, preferably 0.005 to 1.0 wt %. When the average particle diameter is smaller than 100 nm or the content is smaller than 0.001 wt %, the winding property and conveyance property in the film formation process of the film become unsatisfactory and blocking readily occurs. When the average particle diameter is larger than 1,000 nm or the content is larger than 5 wt %, the effect of particles projecting to the surface on the coating layer A side becomes remarkable and electromagnetic conversion characteristics degrade.
The base film C of the present invention can be produced by methods that are conventionally known per se.
Taking a biaxially oriented polyester film as an example, when the base film C is a single-layer film, the polyester resin is extruded into a film from a nozzle at a melting point of Tm.degree. C. to (Tm+70).degree. C. and quenched at 40 to 90.degree. C. to give an unstretched film. This unstretched film is stretched to 2.5 to 8.0 times, preferably 3.0 to 7.5 times, in a monoaxial direction (longitudinal or transverse direction) at a temperature of (Tg-10) to (Tg+70).degree. C. (Tg: glass transition temperature of resin used) in accordance with a commonly used method. Thereafter, coating solutions for forming the coating layer A and coating layer B are each applied to both sides of the film, and the film is stretched to 2.5 to 8.0 times, preferably 3.0 to 7.5 times in a direction perpendicular to the above direction at a temperature of Tg to (Tg+70).degree. C. Further, the film may be stretched again in the longitudinal and/or transverse direction as required. That is, 2-stage, 3-stage, 4-stage or multi-stage stretching may be carried out. The total stretch ratio is generally 9 times or more, preferably 12 to 35 times, more preferably 15 to 32 times in terms of area stretch ratio. Subsequently, the biaxially oriented film is heat-set and crystallized at a temperature of (Tg+70) to (Tm-10).degree. C., 180 to 250.degree. C. for example, to provide excellent dimensional stability. The heat setting time is preferably 1 to 60 seconds.
When the base film C is formed by a coextrusion method, two types of polyester resins are laminated together in a molten state in the nozzle or before the nozzle (the former is generally called "multi-manifold system" and the latter "feedblock system") and coextruded to form a double-layer unstretched laminate film having an appropriate thickness ratio, which subsequently undergoes the same steps as of the single-layer film. A biaxially oriented composite film having excellent interlayer adhesion is obtained by this method.
In the production of a composite film, additives such as a stabilizer, colorant, resistivity-adjusting agent (antistatic agent) for a molten polymer, and the like may be added to the polyester resin as required.
The composite polyester film of the present invention can be formed into a deposited magnetic recording medium for high-density recording which has excellent electromagnetic conversion characteristics such as output at a short-wavelength range, SIN and C/N, few dropouts and a small error rate by forming a ferromagnetic metal thin film layer made from iron, cobalt, chromium or an alloy or oxide mainly composed thereof on the surface of the coating layer A by vacuum deposition, sputtering, ion plating or the like, a protective layer made from diamond-like carbon (DLC) and a fluorine-containing carboxylic acid-based lubricant layer on the surface of the ferromagnetic metal thin film layer according to purpose or application, or as required, and a known back coat layer on the surface of the coating layer B. This deposited magnetic recording medium is extremely useful as a tape medium for Hi8 for analog signal recording, and digital video cassette recorder (DVC), data 8 mm and DDSIV for digital signal recording.
The composite polyester film of the present invention can be formed into a metal coated magnetic recording medium for high-density recording which has excellent electromagnetic conversion characteristics such as output at a short-wavelength range, S/N and C/N, few dropouts and a small error rate by uniformly dispersing iron or needle-like magnetic fine powder mainly composed of iron into a binder such as polyvinyl chloride or vinyl chloride-vinyl acetate copolymer, applying the obtained binder to the surface of the coating layer A to ensure that the thickness of a magnetic layer is to be 1 .mu.m or less, preferably 0.1 to 1 .mu.m, and further forming a back coat layer on the surface of the coating layer B by a known method. A non-magnetic layer containing titanium oxide fine powder may be formed on the coating layer A as a layer underlying the metal powder containing magnetic layer as required by dispersing the titanium oxide fine powder into the same organic binder as that for the magnetic layer and applying the obtained binder to the coating layer A. This metal coated magnetic recording medium is extremely useful as a tape medium for 8 mm video, Hi8, .beta.-cam SP and W-VHS for analog signal recording and digital video cassette recorder (DVC), data 8 mm, DDSIC, digital .beta.-cam, D2, D3 and SX and the like for digital signal recording.
Further, the composite polyester film of the present invention can be formed into a coated magnetic recording medium for high-density recording which has excellent electromagnetic conversion characteristics such as output at a short-wavelength range, S/N and C/N, few dropouts and a small error rate by uniformly dispersing needle-like magnetic fine powder such as iron oxide or chromium oxide or lamellar magnetic fine powder such as barium ferrite into a binder such as polyvinyl chloride or vinyl chloride-vinyl acetate copolymer, applying the obtained binder to the surface of the coating layer A to ensure that the thickness of a magnetic layer is to be 1 .mu.m or less, preferably 0.1 to 1 .mu.m and further forming a back coat layer on the surface of the coating layer B by a known method. A non-magnetic layer containing titanium oxide fine powder may be formed on the coating layer A as a layer underlying the metal power containing magnetic layer as required by dispersing the titanium oxide fine powder into the same organic binder as that for the magnetic layer and applying the obtained binder to the coating layer A. This oxide coated magnetic recording medium is useful as a high-density oxide coated magnetic recording medium for data streamer QIC for digital signal recording.